Restriction endonucleases are enzymes that occur naturally in certain unicellular microbes—mainly bacteria and archaea—and that function to protect these organisms from infections by viruses and other parasitic DNA elements. Restriction endonucleases bind to specific sequences of nucleotides (‘recognition sequence’) in double-stranded DNA molecules (dsDNA) and cleave the DNA, usually within or close to the sequence, generating DNA fragments of various sizes. In vivo, the restriction fragments in turn serves as the substrates for further exonuclease digestion, leading to total degradation. Restriction endonucleases commonly occur with one or more companion enzymes termed modification methyltransferases. Methyltransferases bind to the same sequences in dsDNA as the restriction endonucleases they accompany, but instead of cleaving the DNA, they alter it by the addition of a methyl group to one of the bases within the sequence. This methylation (‘modification’) prevents the restriction endonuclease from binding to the cleavage sequence, rendering the site resistant to cleavage. Methyltransferases function as cellular antidotes to the restriction endonucleases they accompany, protecting the cell's own DNA from destruction by its restriction endonucleases. Together, a restriction endonuclease and its companion (cognate) modification methyltransferase(s) form a restriction-modification (R-M) system.
A large and varied number of restriction endonucleases have been classified as ‘Type II’ restriction endonucleases. These enzymes cleave DNA at defined positions, and in purified form can be used to cut DNA molecules into precise fragments for gene cloning and analysis. The biochemical precision of Type II restriction endonucleases exceeds anything achievable by chemical methods, making these enzymes the reagents sine qua non of molecular biology laboratories. In this capacity, as molecular tools for gene dissection, Type II restriction endonucleases have had a profound impact on the life sciences in the past 33 years, transforming the academic and commercial arenas, alike. Their utility has spurred a continuous search for new restriction endonucleases, and a large number have been found. Today more than 221 Type II endonucleases specificities are known, each possessing different DNA cleavage characteristics (Roberts, et al. Nucl. Acids Res. 33:D230-D232 (2005)). (REBASE®, rebase.neb.com/rebase). Concomitantly, the production and purification of these enzymes has been improved by the cloning and over-expression of the genes that encode them in non-natural production strain host cells such as E. coli. 
Since the various restriction enzymes appear to perform similar biological roles, in much the same ways, it might be thought that they would resemble one another closely in amino acid sequence and behavior. Experience shows this not to be true, however. Surprisingly, far from resembling one another, most Type II restriction enzymes appear unique, resembling neither other restriction enzymes nor any other known kind of protein. Type II restriction endonucleases seem to have arisen independently of one another for the most part during evolution, and to have done so hundreds of times, so that today's enzymes represent a heterogeneous collection rather than a discrete family. Some restriction endonucleases act as homodimers, some as monomers, others as heterodimers. Some bind symmetric sequences, others asymmetric sequences; some bind continuous sequences, others discontinuous sequences; some bind unique sequences, others multiple sequences. Some are accompanied by a single methyltransferase, others by two, and yet others by none at all. When two methyltransferases are present, sometimes they are separate proteins, at other times they are fused. The orders and orientations of restriction and modification genes vary, with all possible organizations occurring. Several kinds of methyltransferases exist, some methylating adenines (m6A-MTases), others methylating cytosines at the N-4 position (m4C-MTases), or at the 5 position (m5C-MTases). Usually there is no way of predicting, a priori, which modifications will block a particular restriction endonuclease, which kind(s) of methyltransferases(s) will accompany that restriction endonuclease in any specific instance, nor what their gene orders or orientations will be.
Great variability exists among restriction-modification systems. Each enzyme is unique in amino acid sequence and catalytic behavior; each occurs in unique enzymatic association, adapted to unique microbial circumstances; and each presents the experimenter with a unique challenge. Sometimes a restriction endonuclease can be cloned and over-expressed in a straightforward manner but more often than not it cannot, and what works well for one enzyme can work not at all for the next. Success with one enzyme is not a predictor of success with another.